Static proportion or "hold-up" is defined as being the volume occupied by one particular phase in a given volume of the well as defined between two right sections.
Fluids coming from a hydrocarbon well are usually multiphase fluids, comprising mixtures of brine and oil, where the term "oil" is applied herein to all hydrocarbons, in particular of the petroleum type. Typically, water forms a continuous phase while oil is dispersed in the form of bubbles or droplets, with the number and size thereof increasing with increasing oil hold-up, which bubbles or droplets may possibly coalesce; at present, the worldwide average for production from a well is about 15% oil to 85% water. If the oil content is high, then the system is inverted and it is oil that constitutes the continuous phase.
Analysis of the production from a well, e.g. to determine the hydrocarbon flow rate profile as a function of depth, relies in particular on knowledge of the respective proportions of the various phases present. A more particular aspect of such analysis is localizing so-called "productive" zones that contain, at least some hydrocarbons, and zones that are completely unproductive, that contain water only, and which it may be appropriate to isolate in order to limit inflow of water. Particularly for a well providing marginal production, it is desirable for such localization to be highly accurate. Furthermore, inflows of oil and of water must be capable of being quantified in reliable manner, even if the oil content is very low, e.g. less than 5%.
To determine the proportions of water and of oil, it is known, e.g. from French patent 1 467 151, to use a gradiomanometer, a device which measures pressure gradient over a given height, which gradient may be considered as being a function solely of the difference in level between the two measurement points and of the apparent density of the fluid. Given the respective densities of the various phases, it is then possible to calculate the various proportions thereof.
By definition, that type of measurement assumes that the density of the aqueous phase is known accurately. Thus, to measure an oil content of 5%, the acceptable error on the value estimated for the density of the aqueous phase must be less than 0.01 grams per cubic centimeter (g/cc). Unfortunately, the salinity of the waters encountered varies over a very wide range. Although it is indeed possible to perform a calibration measurement by placing the gradiomanometer in a zone where the oil content is nil, e.g. at the bottom of the well, that nevertheless assumes that the calibration water is the same as the water that flows into the well, and that is not always true given the various kinds of "pollution" that can stem from stagnant drilling muds or from inflows of water having different degrees of salinity. For want of good calibration, oil inflows are not located with the desired accuracy.
Further, it is known that measurements by a gradiomanometer are affected whenever production flow rates are very high (friction effects) or whenever flows are not steady (constrictions). Finally, given the principle on which it operates, it is clear that a gradiomanometer is not suitable for performing measurements in wells that are highly deviated or horizontal.
Determining hold-up can also be performed by sampling, e.g. by measuring the variation in the capacitance of a capacitor placed in the flow or by irradiation using photons. However that suffers from the same drawback as a gradiomanometer: the values for water and for oil must be known accurately.
Another approach consists in taking measurements by means of local sensors that produce signals having different levels depending on which phase is in contact with the sensor. U.S. Pat. No. 3 792 347 (Hawley) thus proposes an electrical type measurement by measuring resistivity. Proposals have also been made to perform optimal type measurement by refracting a light ray at the end of an optical fiber or radiofrequency type measurement by measuring dielectric constant (German patent application 2 558 588, French patent applications 2 637 089, 2 645 901, or 2 675 202).
The term "local" is validly applied to a measurement only if the zone of fluid being analyzed at a given instant by a local sensor is small relative to the objects being measured, in this case bubbles dispersed in the continuous phase. In addition, the response of the sensor in a given phase must be stable, and ideally independent of parameters such as chemical composition of the measured phase (e.g. salt content), temperature of the fluid, flow rate of the fluid, etc., for example. This implies, in particular, considerable contrast between the resulting signals. In addition, the disturbance made to the flow by the presence of the sensor must be as small as possible.
If such ideal conditions are in fact achieved, then the hold-up value of a given phase in the stream of fluid passing the probe is quite simply equal to the ratio between the sum of the time periods during which said phase has been detected by the probe divided by the total duration of the measurement.
Although highly attractive in theory, that approach nevertheless suffers from a major difficulty, namely that of performing a measurement by contact while not disturbing the fluid flow. To operate properly, the probe must penetrate into a bubble of oil without deflecting it or deforming it, and in addition the "active" portion of the probe must retain no trace of the bubble of oil once it has moved on downstream from the probe.
In fact, experiments performed using probes known in the art have turned out to be unsatisfactory, in particular because of the poor reliability of the probes and also because of insufficient accuracy, generally with oil contents being overestimated. Far from satisfying the theoretically expected binary characteristics, the signals present major defects, e.g. such as relatively long water/oil transition zones, a decrease over time in the level that corresponds to water, and fluctuations in the levels that correspond both to water and to oil.